SHRIMP in situ isotopic analyses of REE, Pb and U in micro-minerals bearing fission products in the Oklo and Bangombé natural reactors: A review of a natural analogue study for the migration of fission products

摘要

In the Precambrian, parts of the Oklo, Okélobondo and Bangombé uranium deposits of the Republic of Gabon, central Africa, functioned as natural fission reactors. Many elements in the Oklo and Bangombé uranium deposits show variations in isotopic composition caused by a combination of nuclear fission, neutron capture and radioactive decay. Isotopic studies provide useful information to understand the behavior of radionuclides in geological media. In our recent work, in situ REE, Pb and U isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a SHRIMP (Sensitive High Resolution Ion Microprobe). The isotopic results of the SHRIMP analyses on micro-minerals found in and around the Oklo and Bangombé natural reactors are reviewed in this paper. The data suggest the selective uptake behavior of (1) Ra into illite, and (2) Pu into apatite, (3) the formation process of secondary minerals bearing fissiogenic REE and depleted U, (4) evidence of nuggets (e{open}-particles) bearing fissiogenic platinum group elements (PGE), and (5) from the U-Pb systematics of highly altered zircons, the redistribution of U and Pb.